Trim tab servomotor control



April 29, 1952 A. M. M CALLUM TRIM TAB SERVOMOTOR CONTROL Filed Dec. 31.,

INVENTOR. HL FIN /77. 177/75 [HLL U/77 qrrog/wsy Patented Apr. 29 1952 UNITED STATES PATENT OFFICE TRIM TAB ssnvomo'ron CONTROL Alan M. MacCallum, Maywood, N. J., assignor to Bendix Aviation Corporation, Teterboro, N. J., a corporation of Delaware Application December 31, 1947, Serial No. 794,882

titude of the craft causing the craft to gain or lose altitude. In flights controlled by automatic pilot systems the flight attitude is normally corrected by the error signals developed by the artificial horizon gyroscope and used to deflect the elevators to compensate for the tail heavy or nose heavy condition of the craft. Trim tab controls are also provided whereby the aircraft is trimmed by the deflection of the tab surfaces. Both methods as presently used for trimming the craft are far from desirable in view of the constant observation of the flight instruments required of the pilot to position the trim tabs for maintaining a level flight attitude. In the case of the deflection of the elevator by the pilot system to trim the craft, the angle of attack of the surface required to trim the craft, will increase the drag thereon reducing the air speed.

An object of my present invention is therefore, to provide an automatic pilot system with means for trimming the craft in flight by positioning the trim tab surfaces in response to the error signals developed by the artificial horizon gyroscope.

Another object of my invention is to provide in an automatic pilot system a trim tab servo control which is operable upon all error signals and in which the servomotor has a high gear ratio.

A further object of my invention is to provide in an automatic pilot system for aircraft a balanced inductive circuit adapted to be unbalanced upon craft deviation from a level flight attitude to operate the trim tab servomotor to deflect the tab surfaces for restoring the level flight attitude.

Still another object of my invention is to provide inan automatic pilot system a trim tab servo control which shall be automatic and positive in its action, relatively inexpensive to manufacture, which shall have a large variety of applications, and yet be practical and efficientto a high degree in use.

Other objects of this invention will in part be obvious, and in part hereinafter pointed out.

In the accompanying drawing forming a part of this specification and in which the single figure is a schematic wiring diagram of one of the various illustrative embodiments of my invention,

the numeral l0 designates an elevator control .1131, 1943 by me among others.

comprises a source of pitch signal herein circuit of an automatic pilot system as-disclosed in a copending application for Automatic Pilot bearing Serial No. 516,488 and filed on December The circuit In illustrated as a take-01f device having a stator winding l2 and a rotor winding ,|3 connected. across a suitable source of alternating current supply, movable with respect to the stator winding I 2 by the precession ofan artificial horizon gyro I4 about its pitch axis; and two servomotors l5 and IE to operate the elevator surface I! and trim tab surface l8, respectively, of'an aircraft in response to the pitch signal voltages of the takeofill.

The'stator winding :2 of the take-off device is 1 connected by'leads is across a resistor 20 forming a part ofthe input circuit of a triode tube 2|.

The resistor 20 is grounded at one end, as at 22,

passengers within the cabin, or'to a change in loading as the engine fuel is consumed, or to other causes, will result in a precession ofthe vertical gyro l4 aboutits pitch axis, causing a relative movement of the rotor winding 3 with respect to the stator. winding i2 of the take-off device A signal voltage will thus be inducedin the stator winding, the phase and amplitude of which is responsive to .the direction and extent of the change in attitude of the aircraft; This signal ,voltage is applied across the resistor 20 and appears on the grid 24 of the triode 2|. The tube 2| will amplify this signal voltage and impress the same as undulating direct current on the dis-v criminator circuit 30.

The phase discriminator circuit 30 comprises two triode tubes 3| and 32 connected in push-pull arrangement to the plate circuit lead 21 and to a suitable source of A. C. supply 33 by way of a transformer 34. The transformer comprises a primary winding 35 and a secondary winding 36 which is center-tapped and grounded as at 31. One end of the secondary winding is connected by lead 38 to the plate 39 of the tube 3|, while the other'end of the winding is connected by a lead!!!) to the plate 4| of-the tube 32. The grids 43 and 44 of the tubes 3| and 32, respectively, are

we may assume that tube 3| will'be. operated,.

while tube 32 operates when the craft is tail heavy.

Connected into the plate circuit lead '38 of tube 3| is a saturable transformer 5|), while a similar transformer 5| is connected into theleadr4|| of the tube 32. comprises a soft iron core (not shown) heavy primary windings 52 and 53 connected in series by a lead 54 and by leads 55, 56 across the voltage source'33, and secondary windings 51 and 58 connected inseries opposed relation by a lead 59; the ends of the secondary windings being connected respectively to the output leads 60 and 6|. Saturating'windings 62 and 63 are also provided, beingconnected into the platecircuit leads 38 and 46, respectively.

When the aircraft is in level'flight, no displace-- ment signals will be induced in the stator wind-- The signal volt-- mg of the take-off device l ages impressed upon the grids 43 and 44 ofthe discriminatortriodes are-thus zero, and no platecurrent due to signal voltages willfiow. The -network described will be balanced. the voltage induced in the secondary winding 51 being equal to, and opposite in phase to the voltage induced in the secondary winding 58. With the induced voltages equal'and opposite, no current will'fiow in the output leads 60 and 6|.

When a change in aircraft attitude occurs, a'

voltage will be induced in the stator winding l2 of the. take-off device I due to the precession of the gyro vertical about its pitch axis. This voltage is'amplified by the triode 2| and impressed on the grids 43 and 44 of the discrim1nator'30. Depending upon the phase of the induced'displacementsignal, tube 3| or 32 will operate. If we assume a nose-heavy condition of the aircraft, the1tube3| will beoperated to provide an undulatingdirect current through the. saturating:

winding62 of the transformer 50. The current: in the. winding62 will saturate the'core ofv the;

transformer; The alternating current induced.

in. the;.secondary winding 51'is reduced'by the: saturated core,.so that thebalance with the volt- 7 age inwinding 58 destroyed, permitting a current to flow in the output leads 50 and 6|. When tube. 32 operates due to a tail-heavy attitude of the aircraft, the output of tube 32 will saturatethe core; of transformer 5| due to the current in the saturating winding'63. A current will again flow in-thebutputleads 60, 6|, thistimeopposite in phase to that flowing during the. nose-heavy"at:- titude of the aircraft.

Connected'into the plate-circuit leads-38 and seriesopposed relation and providing output" leads 1! and I2, and saturatingwindings-H and Eachof the saturable transformers.

14 being series connected into the leads 38 and 40 with the windings 62 and 63, respectively.

The elevator servomotor I5 is a two phase induction motor having an armature I5 suitably coupled to the elevator l1 and adapted to drive a follow-up signal generator 16, more fully described below. The motor I5 is provided with a fixed phase 11 connected across a suitable source of alternating potential, and a variable phase 18 connected to the leads 60, 6| of the reactors 50, 5|. The trim tab servomotor I6 is similar to motor |5 and is provided with an armature I9 coupled, to the. secondary surface |8 through a high gear ratio,- a fixed phase connected across as'uitable voltage supply, and a variable phase 8| connected across the leads 1 12 of the saturable transformers 65, 66.

A deviation from level flight attitude by the aircraft will unbalance the transformers 50, 5| and 65, 66 in the manner described to provide power-to the servomotors l5 and I6 through the respective leads 63, 6| and H, 12. The operation of the motors l5 and I6 will deflect their respec tive control surfaces to trim the craft. It will benoted that the motors I5 and I6 operate in oppoacross theresistor 23 of the amplifier 2|. Opera-- tion of the servomotor l5 will cause a signal to be generated by the generator IE-which is-im-- pressed across the resistor 23 in opposition to the error signal impressed across the resistor 20. The

two signals tend to cancel each other by the operation of theservomotor I5. The motor I5 positioning the elevator IT in response to the error voltage reduces-the error voltage upon'response of the aircraft to the change in elevator. The follow-up signal will eventually equal the error signal thereby wiping itout, leaving but the follow-up signal on the grid 24 to reverse the motor |5to return the elevator to aposition in which level flight is maintained. 7

No follow-back isprovided for the trim ta The operation of the motor l6- servomotor I6. is dependent upon the power supplied thereto by the leads 7|, 72; When the motor |5 stops, the motor IE will Stop leaving the trim tab |8 in whatever'position it happens to be in at the moment transformers 65, 65 are rebalancedand cut off the power in leads H, 12. The trim tab is thus left in a position in which it continuously acts on thecraft to maintain the level flight attitude.

The reverse operation of the-trim tab motor IS with respect to the elevator servo will aid in" thepositioning'of the elevator'surface thus .reducing thetorque' required of the-motor |5. The large gear reduction of the trim tab servo and the operation thereof on all error signals to move the trim tab l8 at a relatively slow angular rate, serves as an integrator which positions the trim tab for the average load required of the primary surface in maintaining level flight.

It will thus be seen that there is provided an automatic pilot system in which the several objects of this invention are achieved and which is well adapted to meet the conditions of practical use.

As various embodiments may be made of the above invention, and as various changes may be made in the embodiment described, it will be understood that all matter herein set forth, or illustrated in the accompanying drawings, is to be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, I claim as new and desire to secure by Letters Patent:

1. In an automatic pilot system for aircraft having a primary control surface and a secondary control surface movable thereon, the combination comprising a source of error signalvoltage responsive to craft deviation from a desired attitude, a balanced voltage device and a second balanced voltage device both adapted to be unbalanced by the error signals of said source, a servomotor connected to one of said devices for positioning the primary control surface, and a second servomotor connected to the other of said devices for positioning the secondary control surface, said servomotors being operable upon unbalance of said devices to position the primary and secondary control surfaces to restore the desired attitude of the craft.

2. In an automatic pilot system for aircraft having a primary control surface and a secondary control surface movable thereon, the combination comprising a source of error signal voltage responsive to craft deviation from a desired attitude, a balanced voltage device and a second balanced voltage device both adapted to be unbalanced by the error signals of said source, a servomotor connected to one of said devices for positioning the primary control surface, a second servomotor connected to the other of said devices for positioning the secondary control surface, said servomotors being operable upon unbalance of said devices, and a follow-up signal generator operable by said first servomotor tending to balance, and unbalance in an opposite direction said voltage devices to position the primary and secondary control surfaces to restore the desired attitude of the craft.

3. In an automatic pilot system for aircraft having a primary control surface and a secondary control surface movable thereon, the combination comprising a source of error signal voltage responsive to craft deviation from a desired attitude, a balanced voltage device and a second balanced voltage device both adapted to be unbalanced by the error signals of said source, a servomotor connected to one of said devices for positioning the primary control surface, a second servomotor connected to the other of said devices and having a high gear ratio for positioning the secondary control surface at a relatively slow angular rate, said servomotors being operable upon an unbalance of said devices, and a followup signal generator operable by said first servomotor tending to balance, and unbalance in an opposite direction said voltage devices to position the primary and secondary control surfaces to restore the desired attitude of the craft.

4. In an automatic pilot system for aircraft having a primary control surface and a secondary control surface movable thereon, the combination comprising a source of error signal voltage responsive to craft deviation from a desired attitude, a balanced voltage device and a second balanced voltage device both adapted to be unbalanced by the error signals of said source, a servomotor connected to one of said devices for positioning the primary control surface, a second servomotor connected to the other of said devices and having a high gear ratio for positioning the secondary control surface at a relatively slow angular rate, said servomotors being operable upon an unbalance of said devices, and a follow-up.

signal generator operable by said first servomotor tending to balance, and unbalance in an opposite direction said inductive device to position the secondary control surface for the average load recontrol surface movable thereon, the combination comprising a source of error signal'voltage responsive to craft deviation from a desired attitude, a first balanced inductive device and a second balanced inductive device both adapted to bexunbalanced by the error signals of said source, a servomotor connected to one of said devices for positioning the primary control surface, and a second servomotor connected to the other of said devices and having a high gear ratio for positioning the secondary control surface at a relatively slow angular rate, said servomotors being operable upon an unbalance of said devices .to position the secondary control surface for the average load required of the primary control surface to maintain the desired craft attitude.

6. In an automatic pilot system for aircraft having a primary control surface and a secondary control surface movable thereon, the combination comprising a source of error signal voltage responsive to craft deviation from a desired position, a first balanced voltage device, a second balanced voltage device connected in series with said first balanced voltage device, both of said devices being connected for unbalance by the error signal of said source, a servomotor connected to one of said devices for positioning the primary control surface, a second servomotor connected to the other of said devices for positioning the secondary control surface, said servomotors being operable upon unbalance of said devices to position the primary and secondary control surfaces to restore the desired position of the craft.

7. In an automatic pilot system for aircraft having a primary control surface and a secondary control surface movable thereon, the combination comprising a source of error signal voltage responsive to craft deviation from a desired position, a first balanced voltage device and a second balanced voltage device, both adapted to be unbalanced by the error signal of said source, a servomotor connected to one of said devices for positioning the primary control surface, a second servomotor for positioning the secondary control surface connected to the other of said devices in a manner reversely of the connection of said first servomotor to said one device, said servomotors being operable upon unbalance of said devices to position the primary and secondary control surfaces in opposite directions to restore the desired position of the craft.

8. In an automatic pilot system for aircraft having a primary control surface and a secondary control surface movable thereon, the combination comprising a source of error signal voltage responsive to craft deviation from a desired position, a first normally balanced inductive device and a second normally balanced inductive device both adapted to be unbalanced by the error signal of said source, an actuating device connected to one of said inductive devices for positioning the primary control surface, a second actuating device for positioning the secondary control surface connected to the other of said inductive dovices, saidaotuating devices being, operable upon unbalance of saidinductive", devices to position the-primary and secondary: control surfaces; to restore-the desired position of the craft.

9; In an automatic pilotrsystem' for aircraft having. aprimary control surface and a secondary controli'surface' movable thereon, the combination comprising asource of error signal voltage responsive to craft deviation from adesiredv position, a: first balanced voltagedevice, a second balanceduvoltage' device connected in series with saidfirstzbalanced;voltagedevice, both of said devices-beingv connected for unbalanceby the error. slgnal of' said source, motor means connected-to;-.both;of .s'aid devicesfor positioning the primary' and? secondary control surfaces, and.

means operated duringpositioning of one of said surfaces for developing a signal in opposition to the error signal of said source.

ALAN M. MACCALLUM.

REFERENCES CITED The followingreferences. are of record in the file of this patent:

UNITED STATES PATENTS 

